1. Field of the Invention
The present invention relates to a method of making breakage-proof or fracture-resistant calcium fluoride single crystals and their use.
2. Description of the Related Art
Single crystals made of calcium fluoride are required as a starting material for optical components used in DUV photolithography because of their ultraviolet radiation transmission properties, among other things. In DUV photolithography radiation sources, such as excimer lasers, which produce radiation with wavelengths less than 250 nm in the deep or far ultraviolet (DUV=deep or far UV), produce fine structure on a semiconductor wafer coated with a photolacquer. In addition, optical components made from the calcium fluoride, such as lenses or prisms, must have a great optical homogeneity. Defects or faults in the calcium fluoride crystals interfere or destroy the optical homogeneity. The calcium fluoride then exhibits stress birefringence. Calcium fluoride crystals with defects, especially those with stress birefringence, are of course unsuitable for optical components made from them. These crystal defects usually comprise foreign atoms, i.e. impurities, which are incorporated into the crystal lattice and thus interfere or perturb the uniformity of the optical properties of the crystal. Therefore single crystals for optical elements should be made from material having the highest possible purity.
Thus JP-A-10-059799 describes, for example, the action of strontium in calcium fluoride crystals. Accordingly the calcium fluoride should not contain more than 1xc3x971018 atoms/cm3 of strontium, so that the optical properties of the calcium fluoride are not impaired. On the other hand, JP-A-09-315815 teaches that the transmission of intense laser light in the UV range by a calcium fluoride crystal is considerably reduced if the calcium fluoride crystal contains from 1 to 600 ppm of strontium and from 1 to 10 ppm of lanthanum and yttrium impurities. This is the case with the above-described usage of these crystals.
Very high purity calcium fluoride single crystals with dimensions in the decimeter range are made according to conventional crystal growth methods, such as the Stockbarger-Bridgman Methods or the VGF (vertical gradient freezing, cooling with a vertical temperature gradient) methods. These large starting crystals are then put in a form required for optical components by cleaving, grinding or the like and the finished optical components are made finally by special treatments, such as surface processing or shaping.
It has now been shown that the fracture or breakage frequency of crystals, especially high purity calcium fluoride crystals, is comparatively high during processing. Not only is the destruction of a nearly finished component in a final processing step a nuisance, but also micro-fractures and dislocations of the crystal lattice arise during processing, which can finally lead to an optical component having reduced optical transmittance.
The fracture energy of the calcium fluoride crystal along the  less than 111 greater than  surface is only 490 mJ/m2, which is very low. Shale or schist has similar low values. For comparison, for example, quartz, has a fracture energy of 4300 mJ/m2.
In order to make optical components, such as lenses, the raw crystalline material is cut, ground and polished. These steps amount to mechanically working on the crystal structure. The optic axis of the lens extends along the surface normal to the  less than 111 greater than  plane in calcium fluoride. Strong shear forces are produced during working of a curved lens surface, which act along the  less than 111 greater than  plane. For that reason the crystal undergoes microscopic as well as macroscopic damage, in the form of fractures and dislocations.
There have been attempts to make lenses with the optic axis extending with an orientation that is somewhat displaced from the normal to the  less than 111 greater than  plane. However when the optic axis deviates from the normal to the  less than 111 greater than  plane the hardening anisotropies appear more pronounced during working, which only increases the problems encountered during working. Additional problems with stress birefringence results when the deviations are even larger.
It is an object of the present invention to provide a method of making a calcium fluoride single crystal of the above-described type with a higher fracture resistance or breakage resistance.
This object and others, which will be made more apparent hereinafter, are attained in a method for making fracture-resistant large-size calcium fluoride single crystals for optical components, wherein said method comprises melting calcium fluoride raw material and subsequently cooling the resulting calcium fluoride melt to form the single crystal by solidification.
According to the invention the calcium fluoride raw material must be doped with or contain between 1 and 250 ppm of strontium.
According to the invention when the calcium fluoride raw material is weakly doped with from 1 to 250 ppm of strontium, preferably 10 to 100 ppm of strontium, the breakage or fracture resistance of the single crystal made from the crystalline raw material is increased by from 10 to 15 percent. Accordingly fewer dislocations and micro-fractures occur when the single crystal is worked or further processed. The strontium is added in the form of its salts, especially halides or also oxides. A fluoride salt is an especially preferred form for the strontium-containing additive or dopant.
It is best when the strontium is added to the calcium fluoride raw material in an amount of from 80 to 140 ppm, usually about 100 ppm. The fracture energy is generally increased to 560 mJ/m2 when these small amounts of strontium, namely 100 ppm, are added. Because of this increase in the fracture energy the losses due to breakage of the crystal during processing are considerably reduced, whereby the overall yield is greatly increased.
An additional increase in fracture energy of, e.g., only about 10 mJ/m2 to 570 mJ/m2 results when the amount of strontium added is increased to 200 ppm.
It has been shown that the optical properties of the calcium fluoride single crystals are negatively influenced by reduced addition of strontium.
One skilled in the art expects that addition of additional impurities, like the Sr compounds, into the calcium fluoride crystal will increase the number of the defects and imperfections in the crystal lattice. However surprisingly in spite of that a definite reduction of fracture occurrences in the crystals was observed when the strontium was added to the calcium fluoride crystal raw material according to the invention as described above. The optical properties of the crystals are stabilized as a result.
Foreign atoms, such as Na, K, Li, Mg and Ba, can be contained as impurities in the calcium fluoride raw material for the single crystals. Also other impurities are present but at levels substantially below 1 ppm. Also the former impurities may be forced to levels below 1 ppm by a special process during crystal growth, in which they are volatilized. This is certainly true for sodium. Sodium however is an element, which is present in human skin in comparatively large amounts. Because the single crystals are always unavoidably processed in the presence of humans, the sodium concentration cannot be reduced, e.g. on the surface, to less than 2 ppm.
Interestingly an addition of sodium or an increase of the sodium content in the calcium fluoride crystal doped with strontium as described above scarcely increases the fracture energy. On the other hand, the strontium compensates for the negative action of the sodium impurities.
The calcium fluoride raw material for making the single crystal can thus contain between about 1 and 10 ppm of sodium as an impurity.
The increase of the mechanical strength of the calcium fluoride crystal by addition of an extremely small amount of less than 250 ppm, preferably less than 200 ppm, usually about 100 ppm, of strontium is very surprising in view of the at least 2 to 3 percent additive amounts of dopants normally required to cause such effects in crystals. An additional important fact, which favors the use of strontium, is that the strontium does not locally collect or is not enriched in the crystal, but is uniformly distributed throughout the crystalline material. The uniform distribution of the strontium is an indispensible requirement for mechanical strength according to the invention.
The invention is further described further with the aid of the following preferred embodiment.